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Consumer Sensors Lower Cost of Fluorescence Lifetime Imaging

BioPhotonicsMar 2016
CAMBRIDGE, Mass. — A fluorescence lifetime system that relies on a consumer-grade time-of-flight sensor and a mathematical model holds promise as a low-cost imaging option for biomedical research and clinical practice.

Fluorescence lifetime imaging (FLI) is a popular method for extracting useful information that is otherwise unavailable from a conventional intensity image, and its biomedical applications include DNA sequencing and cancer diagnosis. Usually, however, it requires expensive equipment, is limited to either frequency- or time-domain modalities, and demands calibration measurements and precise knowledge of the illumination signal.

Massachusetts Institute of Technology researchers have developed a biomedical imaging system that harnesses an off-the-shelf depth sensor. The coloration of these images depicts the phase information contained in six of the 50 light frequencies the system analyzes. Courtesy of the researchers.
"The theme of our work is to take the electronic and optical precision of this big expensive microscope and replace it with sophistication in mathematical modeling," said Ayush Bhandari, a graduate student at the Massachusetts Institute of Technology and one of the system's developers. "We show that you can use something in consumer imaging, like the Microsoft Kinect, to do bioimaging in much the same way that the microscope is doing."

FLI depends on the tendency of fluorophores to absorb light and then re-emit it a short time later. For a given fluorophore, interactions with other chemicals shorten the interval between the absorption and emission of light in a predictable way. Measuring that fluorescence lifetime interval in a biological sample treated with a fluorescent dye can reveal information about the sample's chemical composition.

In traditional FLI, the imaging system emits a burst of light, much of which is absorbed by the sample, and measures how long it takes for photons to strike an array of detectors. The shorter the light bursts, the more precise the measurements.

Fluorescence lifetimes pertinent to biomedical imaging are in the nanosecond range, and traditional FLI uses light bursts that last just picoseconds.

Off-the-shelf depth sensors like the Kinect — designed to be paired with the Xbox video game console — use light bursts that last tens of nanoseconds. Such a system would appear to be too coarse-grained for FLI.

However, the researchers were able to devise a cost-effective method for estimating lifetimes by repurposing the sensor and applying a mathematical theory that unified time- and frequency-domain approaches. Their system allowed for interpretation of a time-based signal as a combination of multiple frequency measurements.

The system took the measurements of incoming light and fit them to a mathematical model of the overlapping intensity profiles of both reflected and re-emitted light. Once it deduced the intensity profile of the reflected light, the system calculated the distance between the emitter and the sample. In this way, and unlike conventional FLI, the approach doesn't require distance calibration.

The depth sensors that the researchers used in their experiments had arrays of roughly 20,000 light detectors each. The most accurate results came when the detector was 2.5 m away from the biological sample.

That setup didn't afford the image resolution that existing FLI microscopes do. While denser arrays of detectors and optics that better control the emission and gathering of light would inflate the cost of the system beyond the Kinect's $100 pricetag, it should still be significantly less than conventional setups.

The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.